This invention relates to the field of microstructure fabrication, and more particularly, to processes and compositions for patterning and depositioning microelectronic substrates.
The present invention relates to masks used for etching of and deposition on substrate surfaces. Microfabrication techniques for making patterns in a surface of a material may be employed in many areas, These techniques ate particularly important in the electronics industry which employs microelectronic devices having ever decreasing component size. Microfabrication techniques typically employ fine line lithography to transfer patterns into a resist to form a mask. The patterns are then transferred to the surface of the substrate by etching.
It is also known to use a monolayer film of colloidal particles as a mask in the patterning process. The colloidal particles are formed as polymeric spheres. The mask is formed by coating a substrate with a monolayer of colloidal particles such that the particles are fixed to the substrate. The colloidal particles may be arranged on the surface in either a random or ordered array. Contacting the colloidal suspension with the substrate and rotating the substrate in a horizontal plane about an axis normal to the surface of the substrate at a sufficient speed yields a densely packed ordered monolayer of colloidal particles. The degree of order in the arrays is dependent upon the particle size, the degree of attraction to the substrate and the velocity of the flow along the substrate surface. Adjusting the surface chemistry of the substrate ensures that the colloidal suspension wets the substrate.
The array of particles serves a lithographic mask, a suitable etching process transferring the random or ordered array to the substrate. The lithographic mask may also serve as a deposition mask. The current techniques rely on the electrostatic attraction between a surface charge on the colloidal particles and a surface charge on the substrate that is different from the surface charge on the colloidal particles.
The use of colloidal particle layer to form a mask is particularly suited for forming field emission tips in a microelectronic substrate. Field emitters are widely used in microscopes, flat panel displays and vacuum microelectronic applications. Cold cathode or field emission based flat panel displays have several advantages over other types of flat panel displays, including low power dissipation, high intensity and low projected cost. In field emission or cold emission displays, a strong electric field liberates electrons from a substance, usually a metal or semiconductor, into a dielectric, usually a vacuum. Field emitters have been extensively studied and are well known in the art.
The shape of a field emitter strongly affects its emission characteristics, sharply pointed needles or ends having a smooth, nearly hemispherical shape being the most efficient shapes. The limitations of lithographic equipment has made it difficult to build high performance, large field emitters with more than a few emitter tips per square micron. It is also difficult to perform fine feature photolithography on large area substrates as required by flat panel display type applications. Hence, previous attempts have been made to use colloidal particle masks to fashion field emitter tips in microelectronic substrates. Those attempts have not been very successful due to the tendency of the colloidal particles to aggregate in groups, leading to an undesired distribution of field emitter tips in the microelectronic substrate.
The present invention overcomes the limitations of the prior art by providing a colloidal suspension containing a plurality of particles in a medium, applying the colloidal suspension to a surface of a substrate such as a microelectronic substrate, and agitating the colloidal suspension to break up any aggregation of particles. In an exemplary embodiment, the colloidal suspension comprises a plurality of polymeric spheres, e.g., polystyrene, polydivinyl Benzene, and polyvinyl toluene, in a suspension medium which comprises deionized water, a resist such as a photoresist, and a solvent such as isopropyl alcohol. The solvent is removed from the suspension medium, such as through evaporative processes, leaving behind a layer of colloidal particles. The particles are well dispersed across the surface of the substrate, the agitation eliminating any aggregations or clumps. The layer of particles serves as a mask for etching or depositioning the substrate. Etching may be performed through conventional chemical means, or through other means such as reactive plasma or ion beam etching. Likewise, depositioning may be performed by conventional depositioning means.
Applying a mechanical vibration to the colloidal suspension is one method suitable for breaking up aggregations of colloidal particles. Applying ultrasonic or megasonic acoustic energy arc also suitable methods for breaking up any aggregations.
Further control over the colloidal particles may be realized by establishing a potential energy gradient across the substrate. Such can be realized by application of a charge to the plurality of particles and the substrate, or through the application of a heat gradient or gravitational gradient across the substrate.
The method and composition are suitable for forming micron and submicron structures on a substrate, for example, the forming of field emitter tips in a microelectronic substrate. The method does not rely on the use of electrostatic attraction for causing the particles to adhere to the substrate at the point where they strike the surface. In fact, the process relies on the agitation of the colloidal particles after they have been applied to the substrate to break up any aggregations of the particles. The method achieves a better dispersion of microelectronic components, such as field emitter tips, over the surface of the substrate, resulting in a device having better performance characteristics.